The differential gear device of a vehicle is used to absorb the differential produced between front and rear wheels and left and right wheels. In order to avoid the situation that a wheel on one side rotates at a high speed under no load due to slipping etc. whereas the driving power is lost in the wheel on the other side, a power transmission apparatus is provided with a slip limitation or restriction function by using a viscous resistance of a high viscous fluiud, which is disclosed, for example, in Japanese Patent Publication No. 58-48779.
Referring to FIG. 7, for example, in a differential apparatus in which power from an input shaft 101, is transmitted through an input gear 103, a gear case 105 and a differential gear 107 and is divided there for transmission to a first transmission shaft 109 and a second transmission shaft 111 arranged on the left and right sides, an operation chamber 113 is formed to sealingly enclose part of the periphery of the second transmission shaft 111 as shown on the right side of FIG. 7.
Formed between the operation chamber 113 and the second transmission shaft 111 are a plurality of disk-shaped resistance plates 117 which are arranged with a short space therebetween perpendicular to the axis through a bush 115 secured to the outer periphery of the second transmission shaft 111.
The resistance plates 117 are alternatively engaged with the outer periphery of the bush 115 and with the inner periphery of the operation chamber 113. The operation chamber 113 is filled with a high viscous fluid such as silicone oil etc.
With this type of apparatus, the slip limitation function of the resistance plates 117 does not become effective under relatively low differential speed conditions, e.g., when the vehicle is traveling along a straight path or negotiating a curve. However, when a wheel on one side slips during running on a rough road, the driving shaft only on this side is subjected to a lower load, so that the input is transmitted to this side only, which will cause fast rotation under no load. In this situation, a rapid relative rotation is caused between the gear case 105 and the second driving shaft 111, a higher resistance force is acted between the resistance plates 117 and the high viscous fluid to limit the slip between the gear case 105 and the second driving shaft 111, thereby providing power transmission between them and preventing one-side torque transmission toward the shaft on the lower load side, making it possible to provide power transmission toward the driving shaft on the higher load side.
However, in such a power transmission apparatus in which the slip limitation function is utilized only by using the viscous resistance of high viscous fluid, the torque transmission capability is determined only by the coefficient of viscosity of the viscous fluid, the size and shape of the resistance plates and the coefficient of surface friction, so that the apparatus becomes large to increase the torque transmission capability. Further, a rapid relative movement between the both shafts causes a resistance force for torque transmission, which is a power transmission allowing slipping to an extent between the both shafts, so that the whole apparatus becomes large due to the small initial torque transmission capability, which is a shortcoming of the power transmission apparatus. In addition, the slip limitation characteristic is substantially constant with reference to the relative movements, which could not be readily changed. Accordingly, it cannot respond to subtle changes in the running conditions.
This invention is created to overcome these problems in the prior art, wherein the viscous resistance of a fluid is utilized with the operation characteristics and torque transmission capacity improved and with the whole apparatus being compact.
Another object of the present invention is to provide a power transmission apparatus which readily responds to variations in the running conditions by arbitrarily adjusting the operation characteristics and torque transmission capacity.